1. Field of the Invention
This invention relates to apparatus for chemical vapor deposition (CVD) on semiconductor wafers and, more specifically, to a shield for shielding the upper edge of a susceptor to prevent deposition thereon.
2. Description of Related Art
During the formation of an integrated circuit structure, various layers of materials are deposited on a semiconductor wafer. One of the processes commonly used to achieve this is chemical vapor deposition (CVD).
As disclosed in U.S. Pat. No. 5,304,248, the disclosure of which is incorporated herein by reference, a typical reactor used for CVD processing of a semiconductor wafer is illustrated in FIG. 1. In this CVD reactor, a wafer 10 is supported on a circular susceptor 11 beneath a gas inlet or shower head 12. Processing/deposition gas containing molecules comprising a metallic cation, such as Tungsten, enters the CVD chamber through the shower head 12 and reacts adjacent to the wafer so that the metal is deposited on the face or top surface of the wafer 10.
A shadow ring 13 overhangs and is supported by the edge of the wafer 10.
The shadow ring 13 defines an exclusionary zone, typically 1.5 to 2.0 mm wide around the perimeter of the wafer, onto which no deposition occurs. The purpose of creating this zone is to exclude deposited materials from this zone, the edge and the backside of the wafer and thereby reduce the generation of unwanted particles in the chamber. This is a particularly important where the deposited material does not adhere to such areas.
The deposition of Tungsten is a good example. Tungsten does not readily adhere to certain surfaces and before it is deposited over silicon oxide on a semiconductor wafer, the oxide surfaces must be pretreated for the tungsten being deposited to properly adhere to the surfaces (e.g. by deposition of a Titanium Tungsten (TiW)) or Titanium Nitride (TiN)). Typically, the edge and backside of the wafer surfaces have not been pretreated and any deposited Tungsten would not adhere properly and tend to flake off in particles.
Furthermore, the exclusionary zone also provides a "buffer" zone which, when contacted by wafer handling equipment, does not readily chip or flake as would a fully/or inadequately processed surface. Such chipping and flaking could also result in unwanted particle generation.
As can also be seen from FIG. 1, a pumping ring 14 is located on a supporting lip or shoulder 15 inside the chamber and has an inner diameter such that an annular gap is defined between the ring 14 and the outer edge of the susceptor 11. During processing, a non-reactive purge gas is introduced, at a pressure greater than the pressure of the deposition gas, into the chamber from a position below the susceptor. This purging gas flows from below the susceptor 11 to the region above it through the annular gap. The purpose of the purging gas flow is to inhibit passage of the deposition gas into the region below the susceptor and, thereby, help prevent unwanted deposition on the surfaces of the elements of the reactor that are located in this region.
The shadow ring 13, as is illustrated in this figure, rests on both the susceptor 11 and the pumping ring 14. As such, it partially obstructs the annular gap, defined between the susceptor and the pumping ring, and thereby further controls the flow of purging gas from below to beneath the susceptor.
The problem with the apparatus illustrated in this figure is that it cannot be used in applications where deposition is to occur over the entire surface of the wafer. This is so even if the shadow ring were removed, because of how a wafer is heated in processing reactors. Deposition is effected by, amongst others, the temperature of the wafer. Typically, the susceptor is heated by means of, for example, heater lamps. The heated susceptor then conductively heats the wafer.
In any heated susceptor a certain amount of heat loss will occur at the edges resulting in a temperature drop off toward the edges of the susceptor, with its perimeter regions being cooler than its central region. As the wafer is heated by conduction, susceptor described above, which is approximately the same diameter as the wafer, heats the wafer less at the edges than at the center. This, in turn, leads to non-uniform and less easily controlled deposition over the entire wafer.
Merely enlarging the susceptor does not solve the problem. An enlarged susceptor would have surface areas that are exposed to deposition. Such deposition must be removed, before the next process cycle commences, by means of a cleaning process that usually is achieved by plasma etching. Although plasma etching is very effective on the quartz and alumina components or surfaces in the chamber, the susceptor is made of anodized aluminum. This material is not as resistive to the plasma etch and would rapidly wear away. Accordingly, any deposition on the susceptor which would need to be removed by plasma etching would be very disadvantageous.
Thus, the need exists for a processing apparatus that allows for processing of the entire wafer's surface and that, at the same time, heats the wafer uniformly. In addition, the apparatus should preferably also minimize edge and backside deposition on the wafer, for the reasons set out above.